In the implementation of high speed logic, it is often desirable to use a Schottky Barrier diode to prevent the saturation of the switching transistor. The degree of improvement is limited however by the nominal barrier height, the variability in barrier height due to the epitaxial doping level, and the series resistance between the Schottky Barrier diode and the collector contact.
Typical prior art formation of Zener diodes on a large scale integrated circuit chip required extra processing steps to achieve a controlled low breakdown voltage comparable to the signal levels on the chip.
In typical prior art applications of Schottky Barrier diodes in large scale integrated circuitry, the Schottky Barrier contact is made at one point on the surface of the semiconductor substrate and serves as the anode for the device. The Schottky Barrier diode contact may be aluminum which is placed directly in contact with an n-type epitaxial layer of silicon. Located at a position proximate to the Schottky Barrier diode contact is an ohmic contact which serves as the cathode of the diode and is formed by placing an aluminum layer in contact with an n+ diffusion in the epitaxial layer. When the anode is biased positively with respect to the cathode, conventional current flows from the Schottky Barrier contact, through the bulk of the epitaxial layer to the negatively biased ohmic contact. A voltage drop associated with the current flow occurs across the Schottky Barrier which is a function of the epitaxial doping and the bulk epitaxy resistance. Thus, the forward biased diode voltage drop for a Schottky Barrier diode is the sum of the potential drop across the junction V.sub.j plus the series resistance voltage drop through the epitaxial layer between the anode and the cathode. Typical n-type epitaxial silicon layers have a conductivity which varies by as much as plus or minus 50%. Thus the contribution of the junction and series resistance voltage drop across the Schottky Barrier diode can vary significantly from one LSI process batch to another. Thus Schottky Barrier diodes have had only limited usage in large scale integrated circuit design in the prior art due to their substantial variability in their resulting characteristics.